In a high power system, it is common to use multiple batteries to provide back-up power when the primary power source becomes unavailable. Sufficient energy storage is especially important in space systems which have to be fully operational in eclipse when solar energy is not available to a solar panel array. When multiple batteries are used, it is essential that these batteries discharge equally so (1) no battery is degraded or damaged due to over-discharge, and 2) battery capacity is utilized effectively.
If discharge current sharing control is not implemented in a multiple battery power system, the consequences will likely result in the following undesirable conditions. (1) Battery degradation or damage caused by over-discharging a battery which can lead to long term degradation or damage. In the case of over-discharging a Lithium-ion battery, the battery will give rise to a permanent short circuit. (2) Ineffective utilization of battery capacity involving the use of a scheme to terminate battery discharging in a nearly over-discharged battery by disabling its associated battery discharge power regulator. The power system is then current-limited by the remaining discharge power regulators. (Note: power regulators typically have a current limit for self-protection).
Several approaches are currently available to solve this problem. One approach is shown schematically in FIG. 1. In an unregulated power system, multiple batteries 10, 12 and 14 are respectively connected together through diodes 16, 18 and 20. Batteries with a higher state of charge (and higher voltage) will discharge first to balance the state of charge between batteries. A disadvantage of this approach is that an unregulated power system places the burden on the load unit to operate over a wide range of bus voltages. This type of power system typically results in higher mass and cost. In addition, when all the batteries do not have the same number of battery cells due to cell failure and cell bypass, the capacity from the battery with lower cell count cannot be utilized to the full extent.
Another approach is shown schematically in FIG. 2. This approach used in a regulated power system to avoid battery over-discharging divides the loads 28, 30 and 32 between batteries 22, 24 and 26 and the associated battery discharge control electronics. In this approach, current sharing is not required. A disadvantage of this approach is that a system in which the loads are divided in load groups and powered by separate batteries typically results in a heavier system. Each battery must be sized to account for the maximum load. Most space or airborne systems carry redundant units to meet reliability requirements. Thus, a power system of this type is sized to power both the primary and redundant units since the primary and redundant units are usually placed in separate load groups to avoid failure propagation.
A third approach is shown schematically in FIG. 3. In this approach, each power regulator is under the control of its associated control amplifier. A primary control amplifier regulates the primary power regulator when the primary power source is available. The battery power regulators are set up in a master-slave configuration. A master battery power regulator 48 including a first discharge control amplifier 50 and a first battery discharge controller 52 controls operation of a master battery 44. Each slave battery power regulator 53, each including a second discharge control amplifier 54 and a second battery discharge controller 55, controls the operation of each of the N plural slave batteries 46, where only one slave battery and slave battery power regulator are shown for simplicity. A current sharing control amplifier is used in each slave unit to force current sharing between batteries. In each slave unit, an error signal from a current sharing amplifier 56 is fed into its discharge control amplifier to adjust the bus voltage set-point. The bus voltage set-point in the master battery discharge controller is fixed. By varying the bus voltage set-points in each slave unit, battery current is forced to be shared among the N slave batteries. A disadvantage of this approach is that separate amplifiers controlling separate power regulators does not allow a very tightly regulated power bus. Primary power sourcing/battery charging and battery discharging in most power systems are typically autonomous without external control. Bus voltage is sensed to determine whether the system should be in primary power sourcing/battery charging mode or battery discharging mode. If each power regulator has its own bus control amplifier and the bus set-points of these regulators are set too close to each other, it will introduce the risk of discharging one battery to charge another battery.
The present invention avoids these problems encountered in the prior art by allowing for battery discharge current sharing in a multiple battery power system while maintaining a tightly regulated power bus. The risk of discharging one battery to charge another battery is eliminated in the inventive system where system mass is minimized such as for use in an aircraft or spacecraft environment.